Conventional maskless exposure systems often use a single laser and scanning optics to deflect the beam towards different positions on the substrate. There are some important side effects of the deflection that usually occur, and that are not always addressed in the different systems. For example, these effects include oblique beam landing which causes position errors on imperfectly flat substrates. In a lithographic system, this may well result in defects in the end product. Other undesired side effects that may lead to such defects are spot growth due to oblique landing of the beam on the substrate and defocusing of spots.
The deflection can for example be done with a double galvanometer mirror. An F-theta objective is then needed to achieve correct focus on all substrate positions. The other two side effects are not corrected this way. Another method is to use a rotating polygon mirror. Again, optics after the scanner are needed, at least to compensate for the defocusing. The optics may as well correct for the oblique landing such that all beams land perpendicularly on the substrate. However, a relatively complex optical system for this is required.
The systems all suffer from a limitation in achievable spot size. Due to the required long throw distance of the beam after deflection (to reach all the substrate area), the spot cannot be made very small (e.g. not smaller than 50 μm depending on factors like distance, wavelength, numerical aperture of optics). Short optical distances, and therefore smaller achievable spot size, may be obtained when using many light sources. This is because the substrate area to be served by each source becomes small. Some systems are known that apply this principle. However, as the above already suggests, many post scan lenses are needed to deal with the above side effects. These lenses can be small, but not very small: each lens needs to be at least the size of the scan length. This is substantially larger than the beam diameter, and substantially larger than the lenses in the optical path before the polygon. Therefore, the use of many laser beams only worsens the situation with respect to the post scan optics, making it more complex.